1. Technical Field of the Invention
The present invention relates to a position detecting device, and particularly to a position detecting device used to detect the position of a control target in a servo device.
2. Description of the Related Art
A servo device (servo circuit) for servo-driving a motor in accordance with an instruction signal from a controller has been hitherto used in a lens driving operation of a television lens, a pan/tilt driving operation of a camera platform for supporting the television camera or the like. The servo circuit uses a position detecting sensor for detecting the position of a control target to be driven by the motor, and a potentiometer or encoder has been hitherto used as such a position detecting sensor. JP-A-7-230031 describes a position detecting device using a potentiometer.
However, in connection with a case where a potentiometer is used as a position detecting sensor and a case where an encoder is used as a position detecting sensor, the servo circuit required to have high-precision control has the following drawbacks, respectively. That is, when the potentiometer is used as the position detecting sensor, a high-precision potentiometer is needed, which increases the cost. Furthermore, even the high-precision potentiometer has a limitation in the linearity of an output voltage to a detected position (detected angle), and thus the precision of the position detection is restricted. Furthermore, when digital processing is needed, it is required to convert the output voltage of the potentiometer to a digital signal by an A/D converter, and thus the A/D converter is also required to have high precision, resulting in increase in cost.
On the other hand, for example when an incremental type encoder (hereinafter referred to merely as encoder) is used as the encoder, it can carry out high-precision position detection cheaply without any drawback imposed on the potentiometer. However, in order to make the count value of the number of pulses output from the encoder the absolute value (the value indicating the absolute value) corresponding to the position, it is required to carry out an original-point returning operation in which a control target is temporarily moved to the end of a movable range when power is turned on and the count value is initialized to zero, or when the encoder is an encoder with a zero position signal (Z-phase), the control target is moved until the zero position signal thereof is detected and the count value is initialized to zero. Therefore, there is a drawback that a time needed for the original-point returning operation is needed when power is turned on.
An absolute type encoder for outputting the absolute value corresponding to the position as in the case of the potentiometer is known as needing no original-point returning operation. When the absolute type encoder is used, it is required to have high resolution, and thus it has a drawback that the cost is increased. Furthermore, there is also a limit in the resolution of the absolute type encoder, and also it needs parallel outputs. Therefore, it has a drawback that there are required cables and input terminals whose numbers correspond to the number of bits. The present invention has been implemented in view of the foregoing situation.